1. Field of the Invention
The invention relates to a circuit arrangement for the wireless transmission of a control signal to the control path of a controllable semiconductor valve and, in particular, a circuit arrangement in which the control signal is used to modulate a high frequency transmitter whose transmitted carrier is received by a high frequency receiver which comprises a demodulator whose output signal is applied to the control path of the semiconductor valve.
2. Description of the Prior Art
In the above-type of circuit arrangement, the control signal is, typically, first applied to a first modulator where it amplitude-modulates a subcarrier signal. The subcarrier signal is then used to modulate, via a second modulator, a high-frequencycarrier signal which, after modulation, is transmitted to the receiver. At the receiver, first the carrier and then the sub-carrier are demodulated and the resultant signal applied to the valve being controlled. A particular circuit arrangement of this type is known from Swiss Pat. No. 413,933.
Typically, the amplitude modulated sub-carrier signalin the above arrangement comprises, per cycle of the control signal, a signal packet of the sub-carrier frequency, which has, for example, a length of 120 electrical degrees and is associated with the ignition state of the semiconductor valve, and zero or currentless interval, which, for example, has a length of 240 electrical degrees and is associated with the cutoff state of the semiconductor valve. Viewed over several cycles, therefore, the amplitude-modulated sub-carrier signal comprises a series of signal packet and currentless intervals. At the receiver, the occurrence of a signal packet indicates that the respective semiconductor valve is to be ignited, while the occurrence of a currentless interval indicates that the respective semiconductor valve is to be extinguished.
During the aforesaid currentless intervals, the interference signals resulting from external or internal disturbances, e.g., interference voltages in the modulator, irradiation from outside the high-frequency transmitter, switching operations, or corona discharges at the semiconductor valves, often lead to an unintended ignition of the semiconductor valve at the receiver.
It is thus a primary object of the present invention to improve the above-mentioned circuit arrangement's immunity to interference.